Ovipositor Inspires New Needle for Surgery

The ovipositor of a parasitic wasp inspires a mechanical engineer and zoologist to design a new steerable needle for surgery.

by Cassie Kelly

January 25, 2019

The same mechanism that enables a parasitic wasp to lay her eggs inside a caterpillar without killing it could one day help surgeons operate deep inside the human brain or reach tumors in parts the body that are currently inaccessible.

The work comes out of a longtime collaboration between Paul Breedveld, a professor of mechanical engineering at Delft University of Technology, and Johan van Leeuwen, a professor of experimental zoology at Wageningen University. The unlikely pair began working together nearly 20 years ago on surgical instruments that were inspired by squid tentacles and are now being commercialized.

Their latest work is a steerable needle that can move through the body, contorting to go around obstacles without breaking or veering off course. Breedveld hopes it will help surgeons navigate the complex tissue formations of the liver or breast and open up new possibilities for saving lives. Breedveld plans to use the needle for the welfare of patients, but the creature that inspired it put it to a more sinister purpose.

A female parasitic wasp punctures live caterpillars using a needle-like appendage called an ovipositor, through which she swiftly distributes eggs right into the caterpillar’s flesh. She must be careful not to kill the caterpillar in the process, since the larvae that hatch from those eggs must feed on the internal fluids of a living caterpillar. When they are finally ready to mature, the larvae burst from the caterpillar, causing a brutal death.

Previous entomologists have found that the ovipositor is made out of three rod-like components that are connected with tongue-and-groove linkages. Together, they form a tube for the eggs to travel down. But, while the structure of the ovipositor made sense to van Leeuwen and his team, its steering capability perplexed them. Using high-speed cameras, they closely watched the wasp as it maneuvered the ovipositor through a translucent gel.

By measuring the individual rod motions in the 30-micrometer thick ovipositor, Van Leeuwen and his team showed that the wasp incrementally moves one or two of the rods at a time. Friction between the third rod and the gel anchors the third rod in space, enabling the other two rods to inch forward.

The needles have a beveled shape at their ends, which grows more pronounced when extend past the stationary needle. This produces asymmetrical forces at their tips, allowing them to curve.

All this allows the wasp to puncture the caterpillar more quickly and requires less outside force. Moreover, she uses the stationary rod to steer her eggs to the precise spot where her eggs — and the caterpillar — will survive.

“We discovered the wasp can actually steer the needle in any direction, making “C” shapes and “S” shapes, without turning its body,” van Leeuwen said. “It was the first account of spatially showing what this little lady is actually doing.”

To further inform their research, van Leeuwen and his team made calculations of the wasp’s motion as she lays her eggs. Breedveld and his team at Delft’s Bio-Inspired Technology Group (BITE) used these findings and calculations to build their steerable needle.

Still, imitating nature has its challenges.

“Biology is two-fold. First, you never know for sure why something was made the way it was because you can’t talk to the designer. Maybe it remains from a previous evolution or maybe it has a purpose that hasn’t fully evolved yet,” Breedveld said. “The second problem is that we use entirely different principles to make and assemble things. That’s because while nature grows things, mankind builds them.”

As Breedveld and his team began to build the needle, they quickly realized that they could not replicate the tongue-and-groove linkage on such a small scale. Breedveld wanted to make the needle thinner than a single millimeter — hypodermic needles range from 12 mm to .10 mm thick — and they simply could not fabricate those linkages on such a small scale.

So, Breedveld made a few modifications. By going with a 7-rod design instead of the wasp’s 3-rod mechanism, Breedveld was able to achieve the same general structure as the ovipositor and still allow for precise maneuverability.

In this reimagined design, six rods made out of a superelastic material are placed in a circle around a seventh stationary rod, which is connected to a small ring that holds all of the rods together. The six moving rods are connected to a small motor, which powers the needle and intricately slides one rod at a time to achieve the same movement as the wasp’s ovipositor.

Each rod moves relative to the motion of the previous rod, maintaining friction with the medium, which in this case is skin-like gel, throughout the entire mechanism. This maintained friction allows the needle to continuously penetrate the skin, basically self-propelling. The asymmetrical forces at the tip make it possible to steer precisely. Because there is no need for outside “pushing” force, the needle could essentially move an infinite length.

Breedveld’s design is cutting-edge and there is still a long road ahead before surgeons can use it. His next step is to give the needle sensory capabilities. The idea is that tiny sensors will allow surgeons to see (or sense) where they are steering as the needle penetrates the body. This will take even further ingenuity, given that this design is so thin and small. Even more challenging is Breedveld’s plan to make the needle even thinner.

For van Leeuwen, the research has only just begun. His team is working on understanding how the insect’s motor system works. With such a complex muscular system on such a minute scale, it will take further observation to truly understand the forces at play inside the parasitic wasp.